An electrical generator used in the field of electric power generation includes a stator core formed by an overlapping stack of thin electrical grade sheets of iron (laminations) which are coated with suitable insulation to electrically insulate each lamination from contiguous laminations. These laminations when properly stacked and aligned are then clamped together under high compression and so arranged to form a series of equally spaced, uniform stator slots. The clamping force may be provided by a series of through bolts and building bolts equally spaced and extending through a series of equally spaced holes located at the mean radius and a second set of equally spaced semi-circular arcs at the outer diameter. The holes at the mean radius contain long stainless steel and electrically insulated bolts, appropriately called through bolts. The bolts on the outer diameter are usually not insulated, but are in some designs, and are referred to as building bolts. Some manufacturers do not ground the stator core and, therefore, the outer clamping members may be insulated as well. The clamped stator core slots contain close fitting, well confined electrical windings consisting of a wound series of bottom coils and top coils, which is referred to as the winding.
The inter-laminar resistance of all the electrical steel stator laminations is measured approximately every five years by the EL-CID (electromotive core imperfection detector) maintenance test. This test requires a temporary loop of wire to be drawn through the air-gap between the stator and non-rotating rotor. The machine must be off-line for the test. The core is then excited at 60 Hz and a coil of wire (test coil) is pulled through the air gap along the inside diameter of the stator core, which is commonly referred to as the tooth top. By means of a test coil and a reference coil, the inter-laminar resistance of each lamination is then evaluated by electronic means that compares the amplitude and phase shift between the EL-CID test coil and the reference coil. This test is very effective at locating electrical shorts between adjacent laminations except for the non grounded stator cores. In that case a lamination would have to have two shorts within its expanse for the EL-CID to detect a problem. With a grounded core a single short can be detected. A second method to evaluate stator core inter-laminar resistance is to remove the rotor, install a very heavy current carrying member at the geometric center, apply a heavy 60 Hz current to this center conductor, which encircles the entire machine, and then energize the conductor in order to excite the core to full magnetic excitation, and finally look for “hot spots” with an infrared imaging device. When a hot spot is found with either method, the inter-laminar clamping force is reduced and wedges are used to separate the shorted laminations enabling mica insulation to be placed between the shorted laminations thereby restoring stator core electrical insulation integrity. Unfortunately, the long time between stator core integrity tests has resulted in too much insulation degradation between inspections with the inevitable core failure, which then requires stripping the winding from the core, a full or partial restack of the stator laminations, followed with a complete rewind of the electrical machine. There is one monitor that is used on hydrogen cooled machines, called a core monitor, which is designed to detect ions given off to the hydrogen gas when the core overheats and throws these ions into the hydrogen cooling gas. However, many areas are not directly exposed to the hydrogen gas, such as the stator coil slot area between radial vents. Also, some units are cooled only with axial vent holes and have no radial vents. This creates a sealed area around all the stator coils within the stator core making ion release from these areas very limited. The core monitor needs to be quite sensitive to be effective and many times it will give a “false alarm.” In some situations, the core monitor will signal a problem, the operator will shut down the machine, open a cover and crawl inside and inspect what is visible. If nothing is visibly overheated or obviously burned, they usually return the unit to service, risking an eventual stator core failure that was there but hidden from view. Other times the machine continues running and the problem is found during the maintenance inspection described above. If stator core alarms and subsequent crawl through inspections fail to isolate the cause, many operators will simply turn off the core monitor and take the risk of an eventual stator core failure.
Another maintenance check that is always done in parallel with the EL-CID is stator core inter-laminar tightness, which involves measuring the residual tension in the through bolts and building bolts. This is done by means of hydraulic torque wrenches or a hydraulic bolt tensioning tool. If the stator core laminations are not sufficiently tight, the magnetic forces will cause stator lamination tooth tops or vent fingers, which are the mechanical spacers in the radial vents, to vibrate. This vibration has in many instances caused pieces of laminations and vent fingers to break off, becoming loose parts, and, as a direct consequence, cause a short to ground within the core. This happens because cooling gas windage in conjunction with the magnetic forces can cause these loose pieces to vibrate and wear through the stator coil electrical stator coil ground-wall insulation, creating the short to ground and possibly significant core damage before the relays and breakers can trip the unit off line.
Stator coil wedge tightness is another maintenance check that is performed in parallel with EL-CID and stator core inter-laminar tightness. It is important to maintain stator coil tightness at a level that prevents any relative movement within any portion of the stator slot. Stator coil movement is often referred to as slot pounding, which can quickly abrade the stator coil electrical ground-wall insulation leading to a short to ground once the stator coil ground-wall insulation gets too thin or cracks through due to the pounding. The tightness of the stator coils is ascertained by one of two methods. If the rotor is removed, the technician taps on all of the stator coil wedges and listens for “hollow” sounds. If the rotor is installed, a robotic device is driven down the air gap and a small hammer taps on each wedge and the resulting vibration is recorded. Above a certain level of vibration, the wedge is considered loose. If more than five consecutive wedges in a line are all considered loose, they have to be retightened, i.e., re-wedged. If these wedges are inboard, outboard wedges must also be removed to obtain access to the loose inboard wedges. The goal of the maintenance effort is to maintain the compressive state between the stator coils and the stator core slot surfaces from one maintenance interval to the next. If a sufficient number and pattern of wedges are loose, a complete re-wedge of the entire electrical machine is required. Also, a stator core wedge can be tight on one end of the wedge and loose at the other end of this same wedge in which case the wedge is judged to be loose. It should also be noted that if a condition of stator coil vibration has been achieved as disclosed in Patent Application Publication US 2008/0036336 A1, stator coil electrically insulating ground-wall insulation has already been seriously compromised. The purpose of the stator wedge tapping test is to ascertain that the wedges are sufficiently tight to prevent any relative motion (vibration) between the stator core slots and the stator coils contained within these slots. Continuous wedge/coil tightness is required for all sections of the stator coils within the stator core not just the sections of stator coil at the ends of the slots as noted in U.S. patent application Ser. No. 12/209,333. Slot end stator coil vibration is undesirable but stator coil vibration within the great bulk of the stator core is also equally undesirable and is currently measured only by performing electrical generator partial disassembly and using the robotic wedge tap device approximately every five years. If additional fiber Bragg gratings were used according to the method of U.S. patent application Ser. No. 12/209,333 a fiber Bragg grating would be required for each ripple peak and valley as shown in FIG. 6 (six total per ripple spring) in order to measure the stator coil local tightness due to a single ripple spring, which is the measurement currently performed by the robotic wedge tapping device. With over 1000 top ripple springs in a single machine and six (6) optical fiber Bragg gratings required per top ripple spring as shown in FIG. 6, the complete ripple spring-by-ripple spring stator coil tightness evaluation by means of individual fiber Bragg gratings would be a cumbersome process. That limitation along with the impracticality of trying to link fibers between adjacent top ripple springs as these ripple springs are assembled by wedging and hammering them to a nearly flat condition would require a separate fiber for each top ripple spring, which is too many fibers to consider as a solution. The top ripple springs are also highly stressed thin (0.8 mm) components subject to cracking which makes cutting grooves into the ripple springs and bonding in fiber Bragg gratings a questionable practice. For all of the above enumerated limitations, the invention herein described is proposed.
An addition to the optical sensing capabilities noted in U.S. patent application Ser. No. 12/209,333, the invention herein describes a method to fully measure the operating temperatures of all electrical connections and winding segments within the electric generator. This is needed in order to augment the vibration data mentioned in U.S. patent application Ser. No. 12/209,333 as a critical electrical connector may start to overheat without significant vibratory change just as an electrical connector might change vibration without a corresponding temperature change. Both data sets, vibration and temperature, are fully required to fully diagnose the electric generator operating parameters and for the entire winding to be adequately protected.
As noted in the Patent Application US 2008/0036336 A1 and U.S. patent application Ser. No. 12/209,333, a distributive fiber optic sensing system is the appropriate system to use considering the high voltages found within the electrical generating machine and the grounding/arcing potential associated with electrically conducting or magnetic materials, such as metallic wires, conduits, and fixtures. The distributive fiber optic systems currently available consist of dense-packed Fiber Bragg gratings, Rayleigh and Ramon back-scattering temperature sensors, and the Brillouin sensors. The Brillouin is further divided into BOTDA, BOTDR, and DPP-DA/Brillouin. These different versions of Brillouin can all measure strain and temperature, some simultaneously, some not. For the purposes of this invention, they are all referred to as Brillouin frequency shift with the understanding that that terminology does not distinguish between the different versions, which is immaterial for the purposes of this invention. Within this collection, the Ramon is primarily for distributive temperature sensing only, whereas the others can quantify both temperature and mechanical strain, when compensation for the combined effect on the sensing fiber optical element(s) due to both temperature and mechanical strain is correctly considered in the construction of the fiber optical element(s). The Raman distributive temperature sensing method is also unique in its utilization of multi-mode fibers, the others primarily employ single mode and/or polarizing maintaining standard optical fibers.